Microsphere handling at the microscale: New tools and new effects

Analytical techniques are essential in detecting components within complex samples retrieved from various applications, e.g., rapidly detecting pathogens to prevent an outbreak. High Performance Liquid Chromatography (HPLC), is an analytical technique, in which a sample is pumped through a packed bed of microspheres (the column). Within this column, the components of the injected sample interact differently with the beads, hence leaving the column at different intervals.

Improving the efficiency of HPLC systems might benefit the clinical diagnostics of diseases, e.g., cancer. One approach to achieve this is by changing the packing of microspheres in the column from a random to an ordered state. We propose to accomplish this goal by a layer-by-layer assembly of these beads. This strategy is pursued by exploring the concept of a vacuum-driven force capturing a monolayer of precisely positioned beads on a micromachined device. As simple as it sounds, it suffices to say that it has been a challenge to assemble the particle monolayer. The main difficulties originated from the aggregation of these beads and their uncontrollable supply under dry conditions.

We have applied various techniques, such as rubbing, high voltage power supply systems, and shakers, to break the large cluster of microspheres, prior to offering them to the experimental setup. Furthermore, we have studied the interaction forces of silica or polystyrene beads on several surfaces to understand the mechanism of why they would stick on surfaces. It was observed that after rubbing, the microspheres unexpectedly had a preference to stick on a Teflon-like material. This result was explained by the tribocharging mechanism, which is the same mechanism responsible for the charging of a balloon while rubbing it on your hair. A key part of the project involved the design and fabrication of devices using micromachining technology. These devices were deployed in several domains of the project: to break the clusters, to control the supply of single particles with a filter, and to capture the particles with the vacuum force. Our studies revealed that droplets carrying the beads enhance the supplement as well as the quality of the obtained particle assembly. Moreover, funnel-like structures on which the microspheres are captured on the device, have proven to enhance the quality of the closely packed assemblies significantly.